1. Field of the Invention
The invention relates to a thin film transistor (TFT) having a circular electrode (hereinafter referred to as a circular thin film transistor (TFT)), a semiconductor device including the circular thin film transistor, and a manufacturing method thereof. It is to be noted that a semiconductor device corresponds to a thin film transistor substrate including a thin film transistor over a substrate, a liquid crystal panel substrate or a liquid crystal module substrate including a thin film transistor and liquid crystals over a substrate, an EL (Electro Luminescence) panel substrate or an EL module substrate including a thin film transistor and a light emitting element over a substrate, a liquid crystal panel including a thin film transistor and liquid crystals over a substrate which are sealed by a sealing material, an EL panel including a thin film transistor and a light emitting element over a substrate which are sealed by a sealing material, a module including an FPC or the like attached to the aforementioned panel, a module including a driver IC connected to an end of the FPC or the like, and a module including a driver IC mounted to the panel by a COG method or the like.
2. Description of the Related Art
A conventional thin film transistor is formed by using a mask such as a photo mask, therefore, a mask is required to be formed for every change in design. Moreover, a thin film transistor is formed by forming a film over the entire surface of a substrate as a mainstream method. For example, in the case of connecting an electrode and a wire formed so as to sandwich an insulating layer, it is required that the insulating layer be partially removed prior to connecting the wire. Furthermore, patterning of an electrode, a wire, and the like requires exposure of a resist to light using a mask, which leads to problems in that the number of steps is increased and that efficiency and environment are both affected by the waste liquid.
It is to be noted that a conventional thin film transistor has each electrode which cross each other, for example, linearly or perpendicularly. In addition, a conventional thin film transistor has each electrode which ultimately has a start point and an end point which do not form a closed loop structure. Therefore, the mechanical and electrical behavior or the like is different between the center and end portions of each electrode. Further, there are many unnecessary areas in the electrode, which ends in occupying a wide area for the thin film transistor. The aforementioned problems are notably present in a thin film transistor having a conventional shape, which is formed by a method using no mask (maskless process) in particular.
In the case of forming a circular thin film transistor having a circular electrode by using a mask as in a conventional way in order to solve the aforementioned problems, it is difficult to form each electrode into a perfect circular shape. Originally, it is difficult to form a perfect curve in forming a circular shape by using a photo mask since the mask is drawn by a laser or the like. Further, cost is increased when the precision is improved. Moreover, when such a photo mask is used to form a resist and etching is performed, corner portions of the resist formed by the photo mask are likely to be etched more (for example, over etching, or the like), which leads to the formation a distorted circular shape. When a source electrode and a drain electrode of a thin film transistor have such irregular shapes, it becomes difficult to form channel portions uniformly, which causes deterioration and variations in characteristics of the thin film transistor.
On the other hand, an ink-jet method has been applied to the field of flat panel displays and actively developed as a manufacturing method using no mask (maskless process), by which a thin film can be patterned without using a mask. An ink-jet method has many advantages in that no mask is required since patterns are directly drawn, application to a large substrate is easy, efficiency of materials is high, and the like, and has been applied to the manufacture of a color filter, electrodes of plasma displays, and the like (For example, see Non-patent Document 1).
However, when such a maskless process is used to form a thin film transistor having a conventional shape, the shape is dependent on an uncertain factor such as a shape of a landed droplet. Therefore, it is difficult to control the shape of an electrode, an active layer, or the like. In this manner, the maskless process has been a major factor to promote the variations in design values which influence the characteristics such as a channel length of a thin film transistor.
[Non-patent Document 1]
T. Shimoda, Ink-jet Technology for Fabrication Processes of Flat Panel Displays, SID 03 DIGEST, p. 1178-p 1181